The present invention relates to a process for manufacturing a metallic component, such as a wheel part for the rolling system of a vehicle, and to such a wheel. The invention relates in particular to a metallic wheel part, such as a wheel disk, made of a light metal, such as aluminum, magnesium or a metal allowing a substantial reduction in weight, such as titanium, or indeed which consists of an alloy of one of these metals.
Wheel disks made of a metallic alloy, such as aluminum, are conventionally manufactured by a forging process or a casting process.
The first of the above-stated processes, although generally producing disks exhibiting satisfactory mechanical and aesthetic properties, has the major disadvantage of entailing high implementation costs.
For several years, preference has been given to the second of the above-stated processes, using a material consisting of an alloy previously converted into a thixotropic, semi-solid metallographic state. This thixotropic state may be characterized by an alloy structure which comprises a non-dendritic primary phase, consisting of globules or nodules substantially spherical in form.
For a description of such a casting process using a thixotropic, semi-solid alloy and of a die for performing this process, reference may be made, for example, to European patent EP-A-710 515.
Reference may also be made to European patent EP-A-439 981 for a description of a method of achieving the above-mentioned thixotropic state.
This forming process is often designated by the generic term xe2x80x9cthixoformingxe2x80x9d, which includes both the principle of die casting (known as xe2x80x9cthixocastingxe2x80x9d) and the principle of casting/forging on a vertical machine (known as xe2x80x9cthixoforgingxe2x80x9d).
Wheel disks formed by this casting process have, in particular, the following advantages, owing precisely to the above-mentioned metallographic state of the alloy, which may be defined by small globules (generally smaller than 120 xcexcm) and distributed in a virtually uniform manner.
These disks may be thinner and consequently lighter than disks obtained from alloys cast in a different state.
Moreover, they are less porous, which results in uniform, improved density and mechanical strength and also in an improved capacity for heat treatment.
This forming process exhibits other advantages, in particular:
a wide range of options with regard to the aesthetic appearance of the wheel disks obtained (with, for example, the possibility of achieving thin walls or considerable variations in section),
a good dimensional precision with regard to the disks obtained, which minimizes machining operations,
high productivity as a result of short cycle times and automation of tasks, and a substantially increased die service life in comparison with that of dies used in conventional processes for injection die casting of liquid metals, such as aluminum.
The object of the present invention is to propose a process for manufacturing a metallic component which consists, in an initial stage, in forming said component of a metallic material in a semi-solid state and having a thixotropic structure, such that said component exhibits better mechanical and weight reduction properties than those stated above.
To this end, the manufacturing process according to the invention includes, in a subsequent cold-treatment stage, in cold-treating at least part of said component by blasting it with projectiles with a view to plastic deformation thereof.
According to one variant of the invention, this manufacturing process includes, in a stage after said initial stage and before said subsequent stage, in subjecting said formed component to structural hardening.
According to a particular embodiment of the invention, said process further includes subjecting said formed component to an intermediate die forging stage after said initial stage, before performing said structural hardening stage.
The process preferably uses, to perform said initial forming stage, an alloy based on a metal belonging to the group comprising aluminum, magnesium, titanium, iron, chromium, cobalt, nickel, copper, zinc, silver, tin, lead and antimony.
Use is advantageously made of an aluminum-based alloy, such as an alloy additionally comprising between 6.5 and 7.5% by weight of silicon and between 0.5 and 0.6% by weight of magnesium.
Such an aluminum-based alloy has the advantage of minimizing corrosion.
According to another feature of the invention, said initial forming stage comprises thixoforming, which may consist of either thixocasting or rheocasting.
It should be noted that the thixocasting process comprises a first stage of filling a die with semi-solid thixotropic metallic material and a second stage of compacting said material under elevated pressure in the die, for example of the order of 100 MPa.
It should also be noted that, in a known manner, the process known as xe2x80x9crheocastingxe2x80x9d essentially comprises mechanical agitation of a liquid alloy to obtain a semi-solid state, then direct casting of the semi-solid alloy obtained, without said casting being preceded by any cooling stage.
According to a further feature of the invention, the structural hardening stage of said process comprises quenching followed by annealing.
According to an exemplary embodiment of the process according to the invention, at least one operation of said cold-treatment stage uses as said projectiles corundum grains between 75 and 150 xcexcm in size.
According to another example of the invention, at least one operation of said cold-treatment stage uses glass microspheres as said projectiles.
According to another example of the invention, at least one operation of said cold-treatment stage uses as said projectiles steel or cast iron pellets between 200 and 800 xcexcm in size.
According to an advantageous feature of the invention, said initial forming stage includes forming a wheel for the rolling system of a vehicle, said wheel comprising a wheel disk and a wheel rim, in such a way that said wheel constitutes said manufactured metallic component and said cold-treatment stage is carried out by blasting with said projectiles all or part of at least one face of said disk and/or said rim.
According to an equally advantageous variant, said initial forming stage consists in forming part of a wheel for the rolling system of a vehicle, said part of a wheel comprising a wheel disk or a wheel rim, in such a way that said wheel part constitutes said manufactured metallic component and said colt-treatment stage is carried out by blasting with said projectiles all or part of at least one face of said disk or said rim.
According to an equally advantageous variant, said initial forming stage consists in forming part of a wheel for the rolling system of a vehicle, said part of a wheel comprising a wheel disk or a wheel rim, in such a way that said wheel part constitutes said manufactured metallic component and said cold-treatment stage is carried out by blasting with said projectiles all or part of at least one face of said disk or said rim.
A wheel according to the invention for a vehicle rolling system, which comprises a wheel rim to which there is fixed, for example by welding, a metallic disk, is such that said metallic disk is obtained by a manufacturing process described above.
The aforementioned features of the present invention, together with others, will be better understood from a reading of the following description of an exemplary embodiment of the invention, given by way of non-limiting example, said description being made with reference to the accompanying drawings.